Subsurface electromagnetic measurements using cross-magnetic dipoles

ABSTRACT

Sensor assemblies including transmitter and receiver antennas to respectively transmit or receive electromagnetic energy. The sensor assemblies are disposed in downhole tools adapted for subsurface disposal. The receiver is disposed at a distance less than six inches (15 cm) from the transmitter on the sensor body. The sensor transmitter or receiver includes an antenna with its axis tilted with respect to the axis of the downhole tool. A sensor includes a tri-axial system of antennas. Another sensor includes a cross-dipole antenna system.

The present application is a divisional of U.S. patent application Ser.No. 10/812,369, filed Mar. 29, 2004, now U.S. Pat. No. 7,239,145.

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates generally to the field of subsurface exploration.More particularly, the invention relates to techniques in whichinstruments equipped with antenna systems having cross-magnetic dipolesare used for improved subsurface electromagnetic measurements andimaging.

2. Background Art

Electromagnetic (EM) induction and propagation logging techniques arewell known in the field of hydrocarbon exploration and production. Atypical EM logging tool comprises a transmitter antenna and one or more(typically a pair) receiver antennas disposed at a distance from thetransmitter antenna along the axis of the tool. The tools are disposedwithin a subsurface formation that has been penetrated by a borehole tomeasure the electrical conductivity (or its inverse, resistivity) of theformation. EM energy emitted from the transmitter interacts with theborehole fluid (“mud”) and surrounding formation to produce signals thatare then detected and measured by the receiver(s). By processing thedetected signal data using inversion algorithms and models well known inthe art, a profile of the borehole or formation properties is obtained.

Geologists and petrophysicists historically have found it necessary tovisually analyze full well cores extracted from zones of interest toassess complex or thinly laminated (also referred to as bedded)reservoirs and aid in the discovery of hydrocarbons. High resolution“microresistivity” measurement techniques have been developed over theyears to contribute to the identification of hydrocarbons in lowresistivity pay zones. High resolution measurements have helped improvethe estimation of reserves in such reservoirs.

Microresistivity tools have been developed for wireline andwhile-drilling applications. Examples of wireline microresistivity toolsinclude the Formation MicroScanner™ tool and the Fullbore FormationMicroimager tool (FMI™) produced by Schlumberger. Logging-while-drilling(LWD) EM tools capable of providing subsurface images are described inU.S. Pat. No. 5,235,285. The '285 patent describes an LWD tool that canmeasure the resistivity at the bit. Examples of tools based on this andrelated principles include the RAB™ (resistivity at the bit) and GVR™(geovision resistivity) tools produced by Schlumberger. These tools arecapable of providing borehole resistivity images of the reservoir rockbeing drilled.

Early microresistivity techniques were implemented for use withconductive muds, usually a mixture of salt water and weighting solids tocontrol mud density. These EM tools have been designed to investigatethe formation beyond the invaded zone present when the well is drilledwith a water-base mud. More and more new wells are now being drilledwith oil-based mud (OBM) containing chemical additives that build andleave a thin impermeable mudcake and usually prevent significantinvasion into the permeable zones around the borehole. Synthetic-basedmuds have also been introduced in the industry. Early microresistivtytools were based on low frequency electrode devices that are not verysuitable for wells drilled with OBM.

Conventional logging tools have been developed to provide subsurfaceimages in wells drilled with OBM. U.S. Pat. Nos. 3,973,181, 6,191,588,and 6,600,321 describe tools capable of imaging operations in OBM. Whileprogress has been made in the development of wireline OBM tools, thedevelopment of EM tools suitable for while-drilling operations in OBMhas been relatively slower. A need remains for improved EM imaging andlogging techniques, particularly in while-drilling applications usingOBMs.

SUMMARY OF INVENTION

The invention provides a tool for determining subsurface properties. Thetool comprises an elongated body having a longitudinal axis and adaptedfor disposal within a subsurface borehole. A transmitter is disposed onthe body and adapted to transmit electromagnetic energy. A receiver isalso disposed on the body at a distance less than six inches (15 cm)from the transmitter and adapted to receive electromagnetic energy. Thetransmitter or receiver comprises at least one antenna with its axistilted with respect to the longitudinal body axis.

The invention provides a method for determining subsurface propertiesusing a tool adapted for disposal within a borehole traversing an earthformation, the tool having an elongated body with a longitudinal axisand including a transmitter and a receiver disposed thereon, thereceiver located at a distance less than six inches (15 cm) from thetransmitter, the transmitter or receiver comprising at least one antennawith its axis tilted with respect to the tool axis. The method comprisesdisposing the tool within the borehole; energizing the transmitter toemit electromagnetic energy; acquiring a subsurface electromagneticmeasurement using the receiver; and determining a subsurface propertyusing the electromagnetic measurement.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a prior art logging-while-drilling system.

FIG. 2 shows a sensor embodiment in accord with the invention.

FIGS. 3A-3C illustrate various antenna arrays of the sensor in FIG. 2.

FIG. 4 shows a sensor antenna constructed on an insulating sheet inaccord with the invention.

FIG. 5 shows a cross-dipole sensor embodiment of the invention.

FIG. 6 shows another cross-dipole sensor embodiment of the invention.

FIG. 7 shows a downhole tubular incorporating multiple sensorembodiments of the invention.

FIG. 8 shows a side view of the vector components and effective magneticmoment of a sensor embodiment of the invention.

FIG. 9 is a schematic diagram of a downhole tubular incorporating asensor embodiment of the invention.

FIG. 10 is a front-view schematic of the sensor configuration of FIG. 9including a shielding mechanism in accord with the invention.

FIG. 11 shows a wedge-shaped sensor embodiment of the invention.

FIG. 12 shows a cross-section of a wedge-shaped sensor of the inventiondisposed within a cavity in a tubular and including a protective shieldfor downhole use.

FIG. 13 is a schematic diagram of shielding mechanism disposed on atubular to cover a sensor of the invention.

FIG. 14 is a schematic diagram of a pair of sensors adapted fortri-axial measurements in accord with the invention.

FIG. 15 shows a logging-while-drilling tool having extendable pistonsthat include sensors in accord with an embodiment of the invention.

FIG. 16A shows a cross-sectional view of a PowerDrive™ tool drilling aborehole.

FIG. 16B shows a sensor embodiment of the invention disposed on adeployable pad of a PowerDrive™ tool.

FIG. 17 shows a downhole tubular including a protruding section housinga sensor of the invention.

FIG. 18 illustrates a technique for finding a dip angle of a dippingplane using sensors in accord with the invention.

FIG. 19 is a flow chart of a method for subsurface imaging in accordwith the invention.

DETAILED DESCRIPTION

EM logging sensors may be based on electrical dipoles (using metallicelectrodes) or magnetic dipoles (using antennas or resonant cavities atVHF). Embodiments of the present invention relate to magnetic dipolesensors for subsurface imaging, the detection of boundaries, faults,fractures, dipping planes, and the determination of borehole walldistances. As used herein, “sensors” is understood to include an EMtransmitter-receiver system. A co-pending application Ser. No.10/674,179, filed on Sep. 29, 2003, entitled “Apparatus and Methods forImaging Wells Drilled with Oil-Based Muds,” by Tabanou et al. andassigned to the present assignee, discloses tools and methods based onelectrical dipole sensors for resistivity measurements in wells drilledwith OBM.

Conventional EM transmitters and receivers consist of coil or loopantennas mounted on a support. A coil carrying a current can berepresented as a magnetic dipole having a magnetic moment proportionalto the current and the area encompassed by the coil. The direction andstrength of the magnetic dipole moment can be represented by a vectorperpendicular to the area encompassed by the coil. Typical downholetools are equipped with coils of the cylindrical solenoid type comprisedof one or more turns of insulated conductor wire. Those skilled in theart will appreciate that the same antenna may be used as a transmitterat one time and as a receiver at another. It will also be appreciatedthat the transmitter-receiver configurations disclosed herein areinterchangeable due to the principle of reciprocity, i.e., the“transmitter” may be used as a “receiver”, and vice-versa. Embodimentsof the invention are suitable for operation at high frequencies (e.g.1-500 MHz, preferably 2-100 MHz, and most preferably around 50 MHz) andcomprise short spacing antenna arrays (e.g. transmitter-receiverspacings on the order of 1 inch [2.54 cm]).

High frequency operation together with short array spacings makes thesensors of the invention capable of providing high-resolution images ofnear borehole regions. Embodiments of the invention take advantage ofmud invasion to provide a sensitive means for detecting geophysicalvariations. Mud invasion provides a relatively uniform background (withrespect to resistivity) in the invaded zone; the relatively uniformbackground makes the detection of small variations easier. Thus, whilemud invasion presents a problem in conventional logging operations,which attempt to derive “absolute” formation resistivities, itfacilitates sensitive, high-resolution well imaging in accordance withembodiments of the invention, making use of “relative” resistivities.

FIG. 1 shows a typical LWD system that includes a derrick 10 positionedover a borehole 11. A drilling tool assembly, which includes a drillstring 12 and drill bit 15, is disposed in the borehole 11. The drillstring 12 and bit 15 are turned by rotation of a kelly 17 coupled to theupper end of the drill string 12. The kelly 17 is rotated by engagementwith a rotary table 16 or the like forming part of the rig 10. The kelly17 and drill string 12 are suspended by a hook 18 coupled to the kelly17 by a rotatable swivel 19. Drilling fluid 6 is stored in a pit 7 andis pumped through the center of the drill string 12 by a mud pump 9 toflow downwardly. After circulation through the bit 15, the drillingfluid circulates upwardly through an annular space between the borehole11 and the outside of the drill string 12. Flow of the drilling mud 6lubricates and cools the bit 15 and lifts drill cuttings made by the bit15 to the surface for collection and disposal. As shown, a logging tool14 is connected to the drill string 12. Signals measured by the loggingtool 14 may be transmitted to the surface computer system 13 or storedin memory (not shown) onboard the tool 14. The logging tool 14 mayinclude one or more sensors of the present invention as describedherein.

Sensors in accordance with embodiments of the invention are designed toprovide EM measurements under a wide range of conditions. FIG. 2 shows asensor 20 embodiment of the invention disposed in a downhole tool 22.The sensor 20 comprises a crossed magnetic dipole transmitter T andreceiver R. The transmitter T comprises two antennas arranged inorthogonal directions such that their magnetic moments are aligned inthe longitudinal (M_(z)) and transverse direction (M_(x)). The receiverR also comprises two antennas arranged in the same orthogonal directionssuch that their magnetic moments are aligned in the same longitudinaland transverse direction (M_(z) and M_(x)). With this sensor, each ofthe two transmitter antennas may be combined with each of the tworeceiver antennas to provide four arrays to measure multiple EMcouplings. These measurements provide the capability to determineformation dips, faults, bedding boundaries, borehole wall distances—evenif the well is drilled with a resistive mud (e.g. OBM). The embodimentof FIG. 2 shows the sensor 20 embedded in a suitable insulating material24 (e.g. high temperature fiberglass composite thermal set or thermalplastic) disposed within a void or cavity 26 formed in the tool body 22.

The operation of sensor 20 in FIG. 2 is best explained with eachtransmitter-receiver array separately. FIGS. 3A-3C illustrate threesimple arrays that comprise components of the sensor 20. Each of thesearrays may be used to provide specific measurements if desired, orcombined to provide measurements for all-mud imaging.

FIG. 3A shows a simple array in which the transmitter antenna T has amagnetic moment (M_(z)) substantially aligned with the direction of thelongitudinal axis of the tool (represented by a dashed line), while thereceiver antenna R has a magnetic moment (M_(x)) substantiallyperpendicular to the longitudinal axis of the tool. The reciprocal arrayconfiguration is shown in FIG. 11. The cross-dipole measurements (i.e.V_(xz)±V_(zx)) obtained with this sensor array provide usefulinformation suitable for OBM imaging. Since typical formations havesedimentation layers with different resistivities, this sensor arraywill detect a signal at a bedding boundary.

Conventional V_(xz) or V_(zx) measurements are generally insensitive tobed boundaries in vertical holes, but they are sensitive to standoff andinvasion. V_(zx)−V_(zx) becomes sensitive to boundaries in verticalholes, but the response is still dominated by standoff and invasion. Onthe other hand, V_(xz)+V_(zx) is less sensitive to standoff and readszero if there is no boundary between the transmitter and receiver.

Measurement of V_(zx)+V_(xz) allows one to detect horizontal bedboundaries. Measurement of V_(zx)−V_(xz) allows for the determination ofthe distance between the sensor array and the borehole wall. Themagnitude of the sensor signal is more pronounced in formations withrelative dips or faults. Thus, this array is particularly suited forimaging bed boundaries in formations with dipping planes. Further, anull reading of this array can be used to confirm the absence of a bedboundary or dipping planes in the measurements obtained using otherarrays.

FIG. 3B shows a simple array having the transmitter antenna T and thereceiver antenna R aligned in the same direction such that theirmagnetic moments (M_(x) and M_(x)) are substantially perpendicular tothe longitudinal axis of the tool. This array provides V_(xx)measurements suitable for horizontal bed boundary and dip detection.With this array, the eddy currents are induced in planes parallel to thelongitudinal axis of the tool. That is, the eddy currents flow up anddown the formations in a vertical well (i.e. across sedimentationlayers). When a bed boundary is crossed, the measurement made with thisarray will produce a discernable response. This is due to the currentdensity discontinuity at the boundary between beds. The sensor will besensitive to resistivity differences in sedimentation layers when it ismoved across a boundary, i.e., the sensitivity region is determined bythe transmitter-receiver spacing. Embodiments of the invention can havearray spacings on the order of 2 inches (5 cm) or less, preferablyaround 1 inch (2.54 cm) or less. This array configuration is sensitiveto bed boundaries regardless of the presence or absence of dippingplanes, provided the adjacent beds exhibit differing resistivities. Themagnitudes of the signals detected by the sensor 20 correspond to theconductivity ratio of adjacent beds.

FIG. 3C shows an array having the transmitter antenna T and the receiverantenna R aligned in the same longitudinal direction such that they bothhave longitudinal magnetic moments (M_(z), M_(z)). This array providesV_(zz) measurements similar to a conventional EM logging tool andinduces eddy currents to flow in loops perpendicular to the longitudinalaxis of the tool. This configuration provides the formation resistivityaround the borehole. Due to the short investigation zone of this array,the resistivity measurements derived from these readings are primarilyaffected by the invasion.

As described above, each array provides useful information in differentsituations. The combination of measurements from the arrays enables thesensor 20 to provide useful information in a wide range of wellboreconditions, mud types, and orientations. The four simple magneticdipoles of the sensor 20 provide four measurements that may berepresented as a voltage matrix V of the form:

$V = {\begin{pmatrix}v_{xx} & v_{xz} \\v_{zx} & v_{zz}\end{pmatrix}.}$

In this notation, the array shown in FIG. 3A provides the v_(xx)component, which is similar to the v_(xz) component that would beprovided by a similar array having a transverse transmitter and alongitudinal receiver. The arrays shown in FIG. 3B and FIG. 3Crespectively provide the v_(xx) and v_(zz) components. The v_(xx)component is sensitive to bed boundaries, regardless of the presence orabsence of dipping planes, while v_(zz) is primarily sensitive toinvasion resistivity (R_(xo)). By having these four magnetic dipoles,the sensor 20 can provide high resolution images of wells as well asinformation on faults, fractures, or dipping planes, and flushed zoneresistivity.

Note that the four measurements at a single borehole location (verticaldepth) may be acquired by selectively energizing the transmitters in thearrays and recording the detected receiver signals (time multiplexing).An alternative approach is to energize two or more transmitters atdifferent frequencies such that the detector signals can bedifferentiated based on frequency (frequency multiplexing). Note that itis also possible to combine the use of time and frequency multiplexingin a single operation. Rotation of a tool (e.g. in LWD) including thesensor 20 provides azimuthal imaging measurements.

The mathematical theory underlying the cross-dipole (e.g. transmitter Tand receiver R of FIG. 3A) measurements of the invention is nowpresented. For a transmitter carrying a current I, the voltage Vmeasured at the receiver can be expressed in terms of a tensor transferimpedance {right arrow over ({right arrow over (Z)}_(RT):

$\begin{matrix}{V = {{Iu}_{R} \cdot {\overset{\overset{\rightarrow}{\rightarrow}}{Z}}_{RT} \cdot {u_{T}.}}} & (1)\end{matrix}$

The transmitter antenna has a magnetic dipole moment oriented along theunit vector u_(T); the receiver antenna is oriented along u_(R). Thetransfer impedance {right arrow over ({right arrow over (Z)}_(RT) hasthe following symmetry property

$\begin{matrix}{\;{{{\overset{\overset{\rightarrow}{\rightarrow}}{Z}}_{RT} = {\overset{\overset{\rightarrow}{\rightarrow}}{Z}}_{TR}^{T}},}} & (2)\end{matrix}$where the superscript ^(T) denotes the transpose tensor.

Two sets of orthogonal unit vectors are introduced, u_(x), u_(y), u_(z)for the formation, and u_(x), u_(y), u_(z), for the tool coordinates,with u along the axis of symmetry of the tool. The z axis isperpendicular to the layers, oriented upward. The tool axis is in thex-z plane. The dip angle is denoted by α, so thatu _(X) =u _(x) cos α+u _(z) sin α,u_(Y)=u_(y),u _(Z) =u _(x) sin α+u _(z) cos α.  (3)

The symmetrized cross-dipole measurement in the tool coordinates can betransformed to formation coordinates as follows:

$\begin{matrix}\begin{matrix}{{V_{ZX} - V_{XZ}} = {{{Iu}_{Z} \cdot {\overset{\overset{\rightarrow}{\rightarrow}}{Z}}_{RT} \cdot u_{X}} - {{Iu}_{X} \cdot {\overset{\overset{\rightarrow}{\rightarrow}}{Z}}_{RT} \cdot u_{Z}}}} \\{= {{{I\left( {{{- u_{x}}\sin\;\alpha} + {u_{z}\cos\;\alpha}} \right)} \cdot {\overset{\overset{\rightarrow}{\rightarrow}}{Z}}_{RT} \cdot \left( {{u_{x}\cos\;\alpha} + {u_{z}\sin\;\alpha}} \right)} -}} \\{{I\left( {{u_{x}\cos\;\alpha} + {u_{z}\sin\;\alpha}} \right)} \cdot {\overset{\overset{\rightarrow}{\rightarrow}}{Z}}_{RT} \cdot \left( {{{- u_{x}}\sin\;\alpha} + {u_{z}\cos\;\alpha}} \right)} \\{= {{I\left( {{\cos^{2}\alpha} + {\sin^{2}\alpha}} \right)}\left( {{u_{z} \cdot {\overset{\overset{\rightarrow}{\rightarrow}}{Z}}_{RT} \cdot u_{x}} - {u_{x} \cdot {\overset{\overset{\rightarrow}{\rightarrow}}{Z}}_{RT} \cdot u_{z}}} \right)}} \\{= {I\left( {{u_{z} \cdot {\overset{\overset{\rightarrow}{\rightarrow}}{Z}}_{RT} \cdot u_{x}} - {u_{x} \cdot {\overset{\overset{\rightarrow}{\rightarrow}}{Z}}_{RT} \cdot u_{z}}} \right)}} \\{= {V_{zx} - {V_{xz}.}}}\end{matrix} & \begin{matrix}\; \\\; \\\; \\\; \\(4) \\(5) \\\; \\\; \\(6)\end{matrix}\end{matrix}$We get the same result in the tool coordinates as in the formationcoordinates. It can be concluded that this measurement is insensitive torelative dip and anisotropy, since the coupling V_(zx)−V_(xz) isinsensitive to it.

The transmitter and receiver antennas of the invention are miniaturizedin comparison to conventional sensors. As such, these sensors may beimplemented on a printed circuit board (PCB). FIG. 4 shows a sensorantenna 28 embodiment of the invention. In this embodiment, a coil 30 isdisposed on an insulating sheet 32 according to the desired pattern toform a flexible circuit. The coil(s) 30 may be formed from any suitableelectrical conductor, including wire or metallic foil. Adhesives (e.g.polyimides, epoxies, and acrylics) may be used to bond the conductor tothe insulating sheet. Alternatively, the coils may be formed by thedeposition of conductive films on the insulating sheet as known in theart. Conductors 34 provide the corresponding electrical connection forenergizing the coil 30.

The insulating sheet can be any electrically nonconductive or dielectricfilm substrate, such as polyimide film or a polyester film having athickness selected to enable bending or flexing as desired. Methods usedto produce the insulating sheet are described in U.S. Pat. No.6,208,031, incorporated herein by reference. Additional antennaconfigurations that may be used to implement the sensors of theinvention are described in U.S. Pat. No. 6,690,170, incorporated hereinby reference.

FIG. 5 shows an embodiment of a crossed-dipole sensor 20 of theinvention. The sensor includes two antennas 36, 38 formed on insulatingsheets, with their magnetic moments (M_(z), M_(x)) having a commonintersection. As noted above, the sensors of the invention may beoperated as transmitters and/or receivers as desired. FIG. 6 showsanother sensor 20 embodiment of the invention. This sensor 20 comprisesan antenna 38 configured similar to that in FIG. 5 and another antenna40 configured on a core or “bobbin” 42 as described in U.S. Pat. No.6,690,170. The magnetic moments (M_(z), M_(x)) of this embodiment have acommon intersection and are orthogonal to one another.

Those skilled in the art will appreciate that the sensor arrays of theinvention may also include bucking antennas to reduce or eliminatemutual couplings between the transmitters and the receivers. The use ofbucking antennas is well known in the art. In one technique, thereceiver's output is set to zero by varying the axial distance betweenthe transmitter or receiver and the bucking antenna. This calibrationmethod is usually known as mutual balancing. U.S. Pat. No. 6,690,170describes mutual balancing configurations that may be implemented withthe invention.

FIG. 7 shows another embodiment of the invention. A downhole toolconsisting of a metallic tubular 22 (e.g. a drill collar) comprisesseveral sensors 20 of the invention disposed in respective cavities 26formed in the tubular. The sensors 20 may be disposed on the tubular 22in various alignments and with differently oriented magnetic dipoles toprovide measurements under many borehole conditions. For example, thisembodiment is suitable for use in vertical as well as horizontalboreholes. Although the sensor magnetic dipoles are not labeled forclarity of illustration, it will be understood that thetransverse-oriented dipoles are represented as dots extending into orout of the page.

Note that FIG. 2 illustrates one embodiment of the invention. Thetransmitter and receiver antennas in this embodiment are arranged insubstantially orthogonal directions. Those skilled in the art willappreciate that other embodiments may be implemented. For example, thesensor 20 antennas need not be arranged in orthogonal directions.Instead, one or more of the longitudinal or transverse antennas may bereplaced with a tilted antenna, which is an antenna having a magneticmoment not parallel to or perpendicular to the longitudinal axis of thetool. FIG. 8 illustrates the vector components of another sensor 20embodiment of the invention comprising a tilted transmitter antenna Tand a tilted receiver antenna R. The tilted antennas providemeasurements that include longitudinal M_(z) and transverse M_(x)components, which may be separated in analysis.

FIG. 9 shows a side view of another sensor 20 embodiment of theinvention. The sensor 20 is disposed in a tubular 22 and includes twotilted receiver antennas R₁ and R₂ (magnetic moments shown as M_(R1) andM_(R2)) and a transmitter antenna T (magnetic moment shown as M_(x)).The transmitter T and receivers R₁ and R₂ of this sensor are arranged toyield a complex voltage measurement that is proportional to theconductivity (or resistivity) of the borehole wall. Note that thetransmitter antenna T and the two receiver antennas R₁, R₂ lie on threeplanes that form a triangle in this view.

In accordance with the sensor embodiments of the invention, thetransmitter T is operable at relatively high frequencies, in a range of1-500 MHz, preferably between 2 and 100 MHz, and more preferably around50 MHz. The dimensions of the sensor 20 are preferably small to providehigh resolution images. For example, in one embodiment the distancesbetween the center of the transmitter antenna T and the centers of thereceiver antennas R₁, R₂ are on the order of 2 inches (5 cm) or less,more preferably on the order of 1 inch (2.54 cm) or less. In someembodiments, the two receiver antennas R₁ and R₂ are connected in seriessuch that their signals are summed during data acquisition. In otherembodiments, the two receiver antennas R₁ and R₂ are independent, andthe signals acquired by these receivers may be combined in the analysisif desired.

As shown in FIG. 9, the magnetic moments of the receivers R₁, R₂ arearranged at angles relative to the magnetic moment of the transmitter T.If both receivers are tilted at the same angle relative to thetransmitter (i.e. forming an isosceles triangle in the side view of FIG.9), then the summation of the receiver magnetic moments (M_(R1) andM_(R2)) results in a moment that is in the same direction as thetransmitter magnetic moment (M_(x)). In this configuration, the sensoris operable similar to a transverse transmitter-transverse receiverarray (e.g. FIG. 3B), making the sensor sensitive to bed boundaries.With its tilted receiver antennas, this sensor is sensitive to dippingplanes, faults, or fractures, particularly if the signals from the tworeceivers are separately processed.

While the sensor 20 shown in FIG. 9 has the two receiver antennas R₁, R₂arranged at the same angle relative to the transmitter antenna T, oneskilled in the art will appreciate that other arrangements are possible.For example, if the two receivers are arranged at different anglesrelative to the transmitter (i.e. forming a non-isosceles triangle inthe side view of FIG. 9), then the summation of the two receivermagnetic moments is equivalent to a magnetic moment of a tilted antenna.In this case, the signals from the “tilted” antenna can be decomposedinto the longitudinal and the transverse components.

FIG. 10 shows the sensor 20 arrangement of FIG. 9 projected behind aFaraday shield 44. The shield 44 includes multiple conductive metalstrips (fingers) 46 interspersed with multiple insulating strips(fingers) 48 to minimize current loops in a conductive tool body 22. Aconductor 50 (e.g. a metallic strip) couples the metal strips 46 at oneend. U.S. Pat. Nos. 6,667,620 and 6,557,794 (both incorporated herein byreference) describe current-directing shields that may be used toimplement the present invention.

The sensors of the invention are not limited to use in any particulartype of subsurface measurement or exploration operation. They may bedisposed within a borehole on any type of support member (e.g. on coiledtubing, drill collars, casing, wireline tools). FIG. 11 shows anothersensor 20 embodiment of the invention. This sensor 20 consists of atransverse transmitter T_(x) antenna and a longitudinal receiver R_(z)antenna disposed in insulating material (e.g. high temperaturefiberglass composite thermal set or thermal plastic) 24 configured in awedge shape.

FIG. 12 shows a side view of a sensor 20, similar to the embodiment ofFIG. 11, disposed in a drill collar 22 within a correspondingly shapedcavity 26. This sensor 20 is configured with a transmitter and receiverincluding four magnetic dipoles, similar to the embodiment of FIG. 2.When implemented for LWD operations, a suitable shield 54 may be placedover the sensor to protect it from the harsh environment. Embodiments ofthe invention may be implemented with metallic shields having slotsfilled with an insulating material and arranged in appropriate patternsas known in the art to prevent the induction of eddy currents on theshield. U.S. Pat. Nos. 6,566,881 and 6,297,639 (both incorporated hereinby reference) describe shield configurations that may be used toimplement the present invention. Some embodiments may also beimplemented with non-metallic shields (e.g. ceramic, Kevlar™, or PEEK™).

FIG. 13 shows another embodiment of the invention including a metallicshield 54 disposed over a sensor 20 of the invention disposed in atubular 22. The magnetic dipole configuration (shown projected behindthe shield) of the sensor 20 is similar to that of FIG. 3A. The shield54 is configured with a series of longitudinal slots 56 to cover thelongitudinal transmitter magnetic dipole (T_(z)) and angled slots 58 tocover the transverse receiver magnetic dipole (R_(x)) to provide EMfiltering. The shield 54 may be affixed over the sensor using anysuitable means known in the art.

FIG. 14 shows other sensor 20 embodiments of the invention. A series ofconductive windings W₁, W₂, W₃ are disposed on the faces of a cube 60 toform a sensor with three orthogonally-oriented magnetic dipoles,commonly referred to as a tri-axial antenna system. FIG. 14 shows atri-axial transmitter T and receiver R. The cubes 60 are formed of asuitable insulating material (e.g. ceramic) and the windings may bedisposed within grooves (not shown) on the cube surfaces. As known inthe art, tri-axial EM antennas may be used for specifically targetedmeasurements and various analysis techniques may be used to derivedesired parameters (See e.g., U.S. Pat. Nos. 6,584,408, 6,556,015). Thecubes 60 are miniaturized (e.g. ¼″×¼″×¼″[0.635^(cm)×0.635^(cm)×0.635^(cm)]) and may be displaced on a tubular(not shown) in very close proximity to one another for the desiredimaging measurements. Wiring and electronics for the sensors 20 may bedisposed in a tubular using techniques known in the art (e.g. viafeedthroughs).

As noted herein, a sensor 20 of the invention is relatively insensitiveto tool standoff effects. If desirable, the tool standoff effects can befurther minimized by deploying the sensor(s) on articulating orextendable devices on the tool body. Deployable pads have beenextensively used in wireline tools to minimize tool standoffs and tomaximize and maintain sensor contact with the borehole wall. FIGS. 15and 16 show two embodiments of the invention incorporating the sensors20 on logging tools equipped with articulating or extendable devices.

Co-pending U.S. patent application Ser. No. 10/605,200, filed on Sep.15, 2003, by Homan et al. (incorporated herein by reference) disclosespressure-compensated pistons for use in while-drilling tools. FIG. 15shows a tool 100 a having four extendable pistons 18 a as described inthis co-pending application. The extendable pistons 18 a arepressure-compensated, by having fluid-filled reservoirs 13 a that arekept at a pressure substantially identical to the pressure outside thetool (e.g. the pressure in a borehole 101 a). The extendable pistons 18a may be deployed, for example, by a bias force from springs behind thepistons. Each extendable piston 18 a includes a pad 19 a, which canhouse one or more sensors 20 of the present invention. The outer surfaceof the pad 19 a is preferably hardened or “hardfaced” with a suitablematerial, as known in the art, to resist wear.

The use of deployable pads in while-drilling tools has been implementedin the PowerDrive™ tool produced by Schlumberger. FIG. 16A shows across-sectional view of a drill collar equipped with three PowerDrive™pads 62 disposed on a PowerDrive™ tool 64 that is in the process ofdrilling a borehole 66.

FIG. 16B shows a sensor 20 embodiment of the invention disposed on oneof the PowerDrive™ pads 62. Although this configuration is shown with asingle sensor 20, other embodiments may be implemented with multiplesensor arrays. The deployable pads 62 may also include other types ofsensors or sources for subsurface measurements as known in the art. Byusing the deployable pads, the sensors 20 can be kept in contact withthe borehole wall to eliminate or minimize standoff effects.

FIG. 17 shows another embodiment of the invention. A downhole tubular 22(e.g. drill collar) is shown with a sensor 20 disposed within a sensorpocket or cavity 26 formed in a protruding section 68 of the tubular.The tubular 22 may be manufactured to include the protrusion 68 andcavity 26 as known in the art. Alternatively, the protrusion 68 may beformed on the tubular 22 by affixing a suitable material to its exteriorwall (e.g. by welding or deposition of a hardened insert). The outersurface of the protrusion 68 is preferably hardfaced with a suitablematerial (e.g. with hardened metal inserts 70) to increase durability.This tubular 22, as well as other tool embodiments comprising thesensors 20 of the invention, is adapted with suitable wiring (e.g. viafeedthroughs) coupled to conventional electronics as known in the art toactivate the sensor (not shown). A downhole tubular 22 equipped with across-dipole sensor of the invention provides a recessed sensor forwhile-drilling applications, with no direct borehole contact required.Other embodiments may be implemented with a plurality of sensor-equippedprotrusions 68 configured as desired (e.g. azimuthally or axiallydistributed about the tubular similar to FIG. 7).

The complex voltage measurement (V) obtained by a sensor 20 of theinvention may be related to the conductivity in the formation andborehole by the following relationship:V=kσ_(apparent),  (7)where σ_(apparent) is the apparent conductivity and κ is the sensorconstant. The apparent conductivity, σ_(apparent), and hence themeasurement, depends on the formation conductivity σ_(f) and the tool orsensor housing (e.g. pad) conductivity. Thus if the tool or padconductivity remains substantially constant, then the “relative”resistivity measurements depend on the formation conductivity σ_(f).Even if the sensor arrays are surrounded by materials that render thehousing/tool conductivity temperature-dependent, the relationship shownin Equation (7) still holds. In this case, the relationship takes aslightly different form:V=κ ₁σ_(f) −C(T),  (8)where κ is replaced by κ₁, a constant that takes into account themechanical and material properties and antenna geometry. The constant κ₁can be found empirically. The second term C(T) is atemperature-dependent term that does not change significantly within ashort distance in a borehole.

In addition to determining bed boundaries, fractures, and faults,embodiments of the invention may also provide information on dip anglesof dipping planes and sensor-borehole wall distances. Embodiments of theinvention may also be implemented with a tool equipped with multiplesensor arrays in configurations that provide measurements with azimuthalinformation (FIG. 18). Dip angles of dipping planes can be derived usingthe azimuthal information and depths of the bed boundaries by fitting asinusoidal curve or using the following equation:

$\begin{matrix}{{\Phi = {\tan^{- 1}\left( \frac{A}{d} \right)}},} & (9)\end{matrix}$where Φ is the dip angle, A is the amplitude of vertical extent of thebed boundary intercepting the borehole, and d is the borehole diameter.In FIG. 18, the dipping bed boundaries on the borehole wall versus thedepth of four sensor arrays labeled 1 through 4 are shown. Asillustrated in the borehole model, the dip plane intercepts the boreholeto produce an oval interception curve. This oval curve shows up aspoints that define a sinusoidal curve (S) in a 2D pad measurement plot.Thus, by fitting the vertical locations of the bed boundaries asmeasured by the four sensors to a sinusoidal curve, or by using Equation(9), the dip angle can be defined. In addition to detecting dip planes,the same technique can also be used to detect a fault or fracture thatintercepts the borehole.

FIG. 19 illustrates a process for determining subsurface properties inaccordance with an embodiment of the invention. A resistivity tool (e.g.a microimager), including a sensor 20 of the invention, is disposedwithin a borehole (step 200). The borehole may contain a resistive mud(e.g. OBM) from the drilling process. A transmitter antenna in thesensor is energized to emit EM energy (step 205). EM measurements aremade using a receiver in the sensor (step 210). In some embodiments ofthe invention, transmitter antennas are energized at different times orat different frequencies to enable measurements of multiple couplings.The measurements may be performed with the sensor in close proximity tothe borehole wall (e.g. with the sensor disposed on a deployable pad, anextendable piston, or a protruding tool section).

Next, the EM measurements are used to determine a subsurface property(step 215). For example, the EM measurements may be used to derive aborehole image, to determine layer boundaries, to determinesensor-borehole distances, and other subsurface parameters as describedherein. As noted above, measurements obtained with a sensor of theinvention provide “relative” resistivities of the formation in the nearwellbore regions. The relative resistivities are obtained with a highfrequency, and most likely in a background invaded by drilling fluids(i.e. relatively uniform background resistivity). Therefore, thesemeasurements will be sensitive to small variations in resistivitiesaround the borehole and suitable for image construction. In accordancewith embodiments of the invention, the transmitter-receiver spacings maybe on the order of 2 inches (5 cm) or less, and preferably on the orderof 1 inch (2.54 cm) or less. In a formation with relative dips,measurements of the invention may also be used to derive the dip anglesof the dipping planes, as noted above.

The present invention provides several advantages. Sensors in accordancewith embodiments of the invention are capable of measuring theresistivities of formations in boreholes drilled with all mud typesincluding OBM and are substantially insensitive to tool standoff orsimilar borehole effects. The sensors measure relative resistivities toprovide high resolution images of the borehole in wells drilled with thevarious types of muds. The measurements can also be used to constructbed-dipping logs. A borehole fracture or fault analysis could also beobtained from such a measurement. The measurements obtained usingsensors of the invention may also be used for geosteering—e.g., to keepthe well path within the pay zone and to avoid crossing a boundary.

When a sensor of the invention is disposed in a conductive housing (e.g.metallic drill collar), undesired EM fields may be induced in thehousing. A mechanism to minimize the induction may be needed.Embodiments of the invention may be implemented with shieldingmechanisms comprising a suitable liner disposed within the sensor cavity(e.g. ceramic or rubber composites, metallic linings) to attenuateundesired EM fields near the sensor as known in the art (not shown). Itis noted that even with a shielding mechanism, some interference betweenthe sensor and the conductive housing may be unavoidable.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having the benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention. For example, the sensorantennas may be configured with multiple dielectric substratesoverlaying one another to achieve modified couplings or to alter themagnetic moments as desired. Other embodiments may be implemented withthe sensors disposed on non-conductive or non-metallic tubulars (e.g.composite tubulars as described in U.S. Pat. Nos. 6,300,762, 6,084,052).

1. A method for determining subsurface properties, comprising: providinga resistivity imaging tool comprising an elongated body having alongitudinal axis and adapted for disposal within a subsurface borehole;a transmitter disposed on the body and adapted to transmitelectromagnetic energy; and a receiver disposed on the body at adistance two inches (5 cm) or less from the transmitter and adapted toreceive electromagnetic energy; wherein the transmitter or receivercomprises at least one antenna with its axis tilted with respect to thelongitudinal body axis, wherein each transmitter and receiver comprisestwo antennas with their axes substantially orthogonal to one another,the transmitter antennas and the receiver antennas being arranged in thesame orthogonal directions; and wherein the transmitter and receiver areboth disposed in an insulating material within a cavity on the elongatedbody; disposing the tool within the borehole; energizing the transmitterto emit electromagnetic energy; acquiring a subsurface electromagneticmeasurement using the receiver; and determining a subsurface propertyusing the electromagnetic measurement.
 2. The method of claim 1, whereinthe disposing the tool includes extending a section of the tool, onwhich the transmitter and the receiver are disposed, toward a wall ofthe borehole.
 3. The method of claim 1, wherein the disposing the toolincludes rotating the tool within the borehole.
 4. The method of claim1, wherein the tool is disposed within the borehole during drilling ofthe borehole.
 5. The method of claim 1, further comprising determiningelectromagnetic couplings between an x-axis receiver antenna and az-axis transmitter antenna.
 6. The method of claim 1, further comprisingdetermining electromagnetic couplings between a z-axis receiver antennaand an x-axis transmitter antenna.
 7. The method of claim 1, furthercomprising determining electromagnetic couplings between an x-axisreceiver antenna and an x-axis transmitter antenna.
 8. The method ofclaim 1, further comprising determining electromagnetic couplingsbetween a z-axis receiver antenna and a z-axis transmitter antenna. 9.The method of claim 1, further comprising determining electromagneticcouplings between the transmitter and receiver to determine one of asubsurface layer boundary, a distance to the borehole wall, or a dipangle of a dipping plane within the subsurface formation.
 10. The methodof claim 9, further comprising using the sum or difference of thecouplings to determine one of a subsurface layer boundary, a distance tothe borehole wall, or a dip angle of a dipping plane within thesubsurface formation.
 11. The method of claim 1, wherein the subsurfaceelectromagnetic measurement consists of an electromagnetic induction orpropagation response of the formation.
 12. The method of claim 1,further comprising determining electromagnetic couplings between thetransmitter and receiver according to:V_(zx)−V_(xz) where V_(zx) is the voltage measured on an x-axis receiverantenna associated with activation of a z-axis transmitter antenna, andV_(xz) is the voltage measured on a z-axis receiver antenna associatedwith activation of an x-axis transmitter antenna.
 13. The method ofclaim 1, further comprising determining electromagnetic couplingsbetween the transmitter and receiver according to:V_(zx)+V_(xz) where V_(zx) is the voltage measured on an x-axis receiverantenna associated with activation of a z-axis transmitter antenna, andV_(xz) is the voltage measured on a z-axis receiver antenna associatedwith activation of an x-axis transmitter antenna.
 14. The method ofclaim 1, further comprising determining the electromagnetic couplingbetween a z-axis transmitter antenna and a z-axis receiver antenna. 15.The method of claim 1, further comprising determining theelectromagnetic coupling between an x-axis transmitter antenna and anx-axis receiver antenna.
 16. The method of claim 1, wherein the receiveror the transmitter comprises a tri-axial antenna system adapted totransmit or receive electromagnetic energy.